I just bought progressive springs and the only part i don't know what to do with is the white PVC tube that comes along, so what am i supposed to do with it?
That is to be used as the preload tube replacing the metal one. Just remember to watch Sean Morleys youtube video on installing the progressive as trimming the preload is not just 1 inch as he explains. Look up vmax fork rebuild as he uses a progressive spring set lowering the forks 1 inch.
There should be instructions in the kit for preload set up. It's something to seriously consider. For me, 1" wasn't really enough. Like everything, personal taste concerning the feel of your suspension differ between us.